<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>圆角演示</title>

    <style>

      .box{width: 100px;
           height: 100px;
           border: 4px solid red;border-radius: 54px; background-color: red;}  /* 本来是50的。但是有边框为4.所以是54*/



    </style>

</head>
<body>
<div class="box">




</div>

</body>
</html>